The invention relates to a method for analyzing Sixe2x80x94Ge alloys, with which a Raman spectrum of a sample is recorded and Raman frequencies and Raman intensities of the Sixe2x80x94Si mode and the Sixe2x80x94Ge mode of the alloy layer are evaluated.
Components on a Sixe2x80x94Ge basis, such as, for example, infrared photodetectors, field effect transistors and photonic mixing detectors, are used, in particular, in optoelectronics. These components are optimized with respect to their properties, such as quantum efficiency at low dissipation power and low noise, essentially via the optimization of the Sixe2x80x94Ge alloy layers present. Optimization means in this respect that an alloy layer grows with a predetermined Ge content, for example, as buffer layer in as relaxed a manner as possible, i.e., is strained as little as possible or grows in a suitable and relaxed way.
It is known from the article xe2x80x9cRaman scattering analysis of relaxed GexSi1-x alloy layersxe2x80x9d of P. M. Mooney et al., Appl. Phys. Lett. 62 (17), 2069 (1993) for the portion of Ge in an alloy layer to be ascertainable via the ratio of the integrated intensities of the Sixe2x80x94Si mode to the Sixe2x80x94Ge mode.
The article xe2x80x9cMeasurements of alloy composition and strain in thin GexSi1-x layersxe2x80x9d of J. C. Tsang et al., J. Appl. Phys. 75 (12), 8098, 1994 describes, in particular, in conjunction with FIG. 6 therein a method as to how the specified intensities can be ascertained.
The Sixe2x80x94Si mode is attributable to phonon excitations on account of Sixe2x80x94Si oscillation movements, the Sixe2x80x94Ge mode to Sixe2x80x94Ge oscillation movements. In this respect, these are LO/TO phonons at k=0 in the crystalline SiGe.
Proceeding on this basis, the object underlying the invention is to improve the method of analysis specified at the outset such that any strain and any Ge portion in an alloy layer can be ascertained in a simple and as exact a manner as possible.
This object is accomplished in accordance with the invention, with the method specified at the outset, in that one or more spectrum contributions lying outside the Sixe2x80x94Ge modes and the Sixe2x80x94Si modes are evaluated as oscillation modes (i.e. vibration modes).
Intermediate modes and/or additional modes in Si cover layers or Si intermediate layers are not, therefore, considered as background in accordance with the invention but rather as a specific spectrum contribution. In accordance with the invention, the Raman spectrum is, when fitted, composed of a plurality of mode lines, namely, in particular, of the Sixe2x80x94Si mode, the Sixe2x80x94Ge mode, the intermediate modes and the cover layer modes and/or intermediate layer modes. As a result, on the other hand, the line profiles relevant for determining the strain via the shift in the Raman frequency and the line profiles required for determining the Ge concentration can be read selectively from the spectrum. Therefore, an optimized peak profile analysis may be carried out by means of the inventive method of analysis in order to obtain profile and position of the Sixe2x80x94Si mode and Sixe2x80x94Ge mode with minimum error.
Furthermore, the influence of lattice dislocations, cover layers and intermediate layers may be ascertained explicitly in order to obtain in this way an exact profile of the Sixe2x80x94Si mode and Sixe2x80x94Ge mode.
Complex Sixe2x80x94Ge alloy layers, which comprise a Si cover layer in addition to a Sixe2x80x94Ge alloy layer or corresponding layer sequences and/or one or more inserted Si intermediate layers which can, in particular, also be strained, may, in particular, be analyzed in accordance with the invention. The Sixe2x80x94Si mode of a cover layer or intermediate layer has a different frequency position to the Sixe2x80x94Si mode of a Sixe2x80x94Ge alloy layer. As a result of the inventive procedure, the profile and the peak position of the Sixe2x80x94Si mode and the Sixe2x80x94Ge mode may be determined with great precision even with the presence of such cover layers or intermediate layers in order to, on the other hand, be able to carry out a concentration analysis of Ge and relaxation determination.
As a result of the inventive method of analysis, a Raman spectrum can, in particular, be evaluated very quickly, i.e., the corresponding results of analysis are available very quickly. As a result, it is possible, on the other hand, to carry out measurements at short time intervals. A layer which has been produced may, in particular, be analyzed instantaneously during a coating process. As a result, it is, again, possible to influence the coating process accordingly in order to obtain an optimized overgrowth of layers on a substrate.
The intermediate modes are, in particular, local Sixe2x80x94Si oscillation modes. The above-mentioned Sixe2x80x94Si mode is brought about by way of Sixe2x80x94Si movement in the Sixe2x80x94Ge alloy. The above-mentioned Sixe2x80x94Ge mode is brought about by way of the Sixe2x80x94Ge movement in the Sixe2x80x94Ge alloy. These are bulk modes, wherein bulk modes are used in this case without any particular designation. The intermediate modes are, in particular, local Sixe2x80x94Si modes which result due to compositional dislocations. The phonon structure is modified by deviations from the perfect crystal lattice or due to defects. They have, in this case, an addition, such as, for example, xe2x80x9clocalxe2x80x9d mode. Cover layers and intermediate layers (in particular, consisting of Si) also have a phonon structure which differs from that of a Sixe2x80x94Ge alloy layer.
The Sixe2x80x94Si mode and the Sixe2x80x94Ge mode are fitted, in particular, by way of an asymmetric curve. An intermediate mode is, on the other hand, fitted by way of a symmetric curve.
A reliable and rapid analysis of a Sixe2x80x94Ge alloy layer may be achieved when a fit spectrum which consists of a plurality of individual fit curves is fitted to the measured spectrum. In this respect, each individual fit curve is, in particular, a symmetric curve and, in addition, it is favorable when each individual fit curve is a Gauss-Lorentz curve. A Gauss-Lorentz curve thereby consists of the product of a Lorentz curve and a Gaussian curve.
It has proven to be advantageous when the Sixe2x80x94Si mode is fitted by way of three individual fit curves while the Sixe2x80x94Ge mode is fitted by way of two individual fit curves.
It has, furthermore, proven to be advantageous when an intermediate mode is fitted by way of a single fit curve.
As a result of such fits a fit spectrum is obtained which has minimal errors, for example, ascertained via a "khgr"2 test, in relation to the measured spectrum. The parts, in particular, of the Sixe2x80x94Si mode and the Sixe2x80x94Ge mode of the alloy may, in particular, be separated out from such a spectrum in order to be able to carry out a rapid and reliable evaluation.
Furthermore, a background is deducted from the measured spectrum in order to eliminate parts of the spectrum not caused by Raman scattering (and, in particular, parts of the spectrum caused by Rayleigh scattering).
A concentration x of Ge in the Sixe2x80x94Ge alloy is determined in accordance with the formula                     I        ⁡                  (                                    S              ⁢                              xe2x80x83                            ⁢              i                        -                          S              ⁢                              xe2x80x83                            ⁢              i                                )                            I        ⁡                  (                                    S              ⁢                              xe2x80x83                            ⁢              i                        -                          G              ⁢                              xe2x80x83                            ⁢              e                                )                      =          A      ⁢                        1          -          x                          2          ⁢          x                      ,
wherein I(Sixe2x80x94Si) is the integrated intensity of the Sixe2x80x94Si mode, I(Sixe2x80x94Ge) is the integrated intensity of the Sixe2x80x94Ge mode and A is a parameter dependent on the Raman spectroscopy device. This formula is also designated as Mooney formula. The Ge content in a Sixe2x80x94Ge alloy layer may be determined by means of this formula from measured parameters, namely the profiles of the corresponding modes, and also optimized accordingly. The profile of the relevant modes may, on the other hand, be determined by the inventive method of analysis in a reliable and exact manner, namely at short time intervals. As a result, an xe2x80x9cin situ determinationxe2x80x9d of the Ge concentration is possible. The degree of relaxation within a Sixe2x80x94Ge layer may, on the other hand, be determined from the Ge concentration ascertained and the ascertainment of the strain via the ascertainment of the shift in frequency of the Sixe2x80x94Si mode.
The parameter A may be determined from comparative measurements, such as SIMS, XRD or EDX. As a result, a specified Raman spectroscopy device may, again, be calibrated in order to facilitate the direct determination of the Ge content of an alloy layer from the intensity ratios measured.
A strain and/or relaxation in the Sixe2x80x94Ge alloy is determined, in particular, by a shift in the Raman frequency, in particular, of the Sixe2x80x94Si mode in relation to a reference frequency. The reference frequency of the Sixe2x80x94Si mode is located at a wave number of 520.8 cmxe2x88x921 and corresponds to the Sixe2x80x94Si mode in the case of unstrained bulk Si material.
The invention relates, in addition, to a method of diagnosing Sixe2x80x94Ge alloys during their manufacture, with which the Raman spectrum is analyzed in a timed sequence during the manufacturing process with the method of analysis in accordance with any one of claims 1 to 17.
In this respect the manufacturing process is controlled, in particular, in accordance with the result of analysis.
In accordance with the invention, the Sixe2x80x94Ge alloy layer is therefore analyzed with respect to strain and Ge content at the same time via the analysis of the Raman spectrum of the Sixe2x80x94Ge alloy layer. Depending on the result, parameters of the manufacturing process can then be fitted in order to optimize the average overgrowth process of layers with respect to the intended use of the semiconductor structure.
The invention relates, in addition, to an apparatus for manufacturing semiconductor layer structures with Sixe2x80x94Ge layers.
In this respect, the object is to provide an apparatus, with which optimized semiconductor components on a Sixe2x80x94Ge basis can be produced.
This object is accomplished in accordance with the invention in that the apparatus comprises:
an epitaxy device for the epitaxial overgrowth of layers with a control device for controlling and/or regulating the manufacture of the layers;
a Raman spectroscopy device for determining the Raman spectrum of a manufactured layer;
alternatively a timer for determining and/or evaluating the Raman spectrum at timed intervals and
an evaluating device, by means of which the Raman spectrum can be evaluated in accordance with the method according to any one of claims 1 to 17.
The manufacturing process of a layer may be carried out by means of such an apparatus on the basis of an analysis of the coating process, i.e., the strain of a Sixe2x80x94Ge alloy layer and its Ge content may be determined in situ. This result may, again, be used to optimize the further build up of the layer in that the coating parameters are fitted accordingly in order to control the further build up of the coating.
It is of advantage, in particular, when the evaluating device is coupled to the epitaxy device such that the manufacture of the layer can be controlled and/or regulated via a result of analysis of the Raman spectrum.
For this purpose, the evaluating device makes one or more control signals available for the control device of the epitaxy device, i.e., the coating procedure may be controlled by means of the corresponding control signals.